Why am I being asked to remortgage when trying to withdraw overpayments on a flexible mortgage?

I have a flexible mortgage with Bank of Ireland from 2004. It's interest only. It was originally from another lender. When I asked to drawdown funds they told me I couldn't because of changes in the law. They quoted the 2008 CCA though they didn't specify which bit. They say the only way to release the money is via a re-mortgage. I'm very annoyed and very suspicious. Can anyone shed any light on this.